1. Field of the Invention
This invention relates to digital-to analog converter circuits, and particularly to circuits that provide a more stable reference signal for a digital-to-analog converter.
2. Description of the Related Art
Accurate, stable and high resolution digital-to-analog converters (“DAC”) are needed for many applications. One application for which they are needed is audio system testing. In particular, they are needed for digital-to-analog converter circuits that are central to an audio signal generator for producing a test signal to be applied to the input of an audio system under test. Since many audio system tests involve comparing the output signal of the audio system to a signal applied to the input of the audio system to determine such characteristics as the linear transfer function, harmonic distortion and intermodulation distortion, the accuracy with which such characteristics can be determined depends on the stability of the input signal.
High resolution and performance integrated circuit (“IC”) DACs with differential current outputs often include an on-chip voltage reference device that is used in conjunction with an external resistor to produce a reference current and thereby set the magnitude of the output currents in response to a given digital input signal. However, for many applications, including but not limited to precision audio testing, the internal voltage reference device is not as stable as is needed. Changes in the operating temperature of the DAC due, for example, to warm up, changes in ambient temperature, and varying heat dissipation caused by varying the sample rate will cause errors in the amplitude of the output signal current.
One approach to this problem is to try to design more stable voltage sources within the DAC itself, but system manufacturers are dependent on IC manufacturers to do so. Accordingly, it would be desirable to provide a digital-to-analog circuit with the stability needed for a particular application that is not dependent on the inherent stability of a component DAC IC.